Add Cd absorber correction

[mdoucet]

Add correction to correct for absorbers.

To test, load the following data in refred (using this version of lr_reduction) and reduce. The two curves should align, with a flat plateau at low Q at about 1:

/SNS/REF_L/IPTS-30101/shared/autoreduce/template_up_cd_small_test.xml

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 76.92308% with 12 lines in your changes missing coverage. Please review.

Please upload report for BASE (next@74dc873). Learn more about missing BASE report.

Files	Patch %	Lines
...n/lr_reduction/scaling_factors/LRDirectBeamSort.py	47.05%	9 Missing ⚠️
reduction/lr_reduction/event_reduction.py	92.85%	2 Missing ⚠️
reduction/lr_reduction/template.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             next      #39   +/-   ##
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  Coverage        ?   82.23%           
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  Files           ?       20           
  Lines           ?     2330           
  Branches        ?        0           
=======================================
  Hits            ?     1916           
  Misses          ?      414           
  Partials        ?        0           

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[backmari]

I downloaded the PR, installed the lr_reduction package in the RefRed conda environment and started RefRed:

conda activate RefRed
cd LiquidsReflectometer/
gh pr checkout 39
pip install .
cd ../RefRed/
python scripts/start_refred.py 

After loading the xml-file in the PR description and reducing in RefRed, the curves align as expected: